Image sensors on a thinned substrate have been designed to improve the colorimetric performance of sensors, enabling them to be backlit via the back side of a very thin silicon layer. This arrangement prevents the dispersion of photons and photogenerated electrons in the substrate and therefore prevents crosstalk, which would impair the colorimetric response since adjacent image pixels correspond to different colors.
The fabrication of an image sensor on a thinned substrate, generally comprising the following steps, starts with a normal silicon substrate with a thickness of a few hundred microns, for industrial handling of collective wafers about ten to twenty centimeters in diameter, this substrate being coated on the front side with an epitaxial layer of single-crystal silicon, optionally isolated from the rest of the substrate by an oxide layer in the case of SOI (silicon-on-insulator) substrates. The electronic circuitry necessary for the various functions of the sensor (essentially image capture) is produced on the front side of this single-crystal layer. The substrate is then bonded, via its front side bearing this circuitry, onto a transfer substrate of sufficient thickness for industrial handling, and the starting silicon substrate is thinned down to a thickness of a few microns. The resulting very small thickness of silicon precludes industrial wafer handling, and this is the reason for the presence of the bonded transfer substrate.
One of the problems that arise in the case of these components is the formation of connection pads for connecting the component to the outside. Mounting the component in a package generally requires connecting wires to be bonded between a metal connection pad provided on the component and metal pads provided in the package.
Because the substrate in which the electronic circuits have been formed is bonded via its front side to a transfer substrate, the front side is no longer accessible. The aim is therefore to establish a connection via the back side by cutting into the thinned substrate, until a metal area that had been formed beforehand during the fabrication steps via the front face is reached.
Notably, the silicon and the insulating layers formed on the front face may be cut into up to the point where there is access to the first aluminum level. A gold connection wire is then bonded by the conventional technique of wire bonding to the bared aluminum area. However, this area is located within a cup since it was necessary to cut into the silicon and the insulating layers that covered it. This excludes the use of what are called “wedge bonding” methods as opposed to “ball bonding” methods, in which the wire (generally made of aluminum) to be bonded arrives too obliquely to be able to be bonded to the inside of a cup. This is why it is necessary to continue using gold wire bonding, even in cases where an aluminum wire would be preferred. In addition, the cup is formed in a semiconductor material, and not in an insulating material, and there are therefore risks of a short circuit between the wire and the edges of the cup.
Moreover, it should also be pointed out that the aluminum areas serving for the wire bonding must in principle be thicker than the aluminum layers that are used for ordinary interconnection functions in the integrated circuit. However, the technique explained above makes it possible in practice to gain access only to the first aluminum level (unless wishing to cut even deeper), but there is no reason for this level to be thick enough to permit bonding. To adapt this solution to an industrial operation, it would therefore be necessary to provide a first aluminum level thicker than that necessary in general, something which would require changing the standard fabrication process, which is not desirable.
This is why a novel method is proposed here for producing connection pads for an electronic component having a thinned substrate.